1. Field of the Invention
This invention relates to an improved process for producing carbon fibers from fibers of polyacrylonitrile. More particularly, this invention relates to an improved process for producing carbon fibers by the pyrolysis of polyacrylonitrile fibers wherein the extended heat treatment heretofore required in order to stabilize the fiber structure so that it may be carbonized can be performed in substantially shorter periods of time.
2. Description of the Prior Art
As a result of the rapidly expanding growth of the aircraft, space and missile industries in recent years, a need was created for materials exhibiting a unique and extraordinary combination of physical properties. Thus, materials characterized by high strength and stiffness, and at the same time of light weight, were required for use in such applications as the fabrication of aircraft structures, re-entry vehicles, and space vehicles, as well as in the preparation of marine deep-submergence pressure vessels and like structures. Existing technology was incapable of supplying such materials and the search to satisfy this need centered about the fabrication of composite articles.
One of the most promising materials suggested for use in composite form was high-strength, high-modulus carbon textiles, which were introduced into the market place at the very time this rapid growth in the aircraft, space and missile industries was occurring. Such textiles have been incorporated in both plastic and metal matrices to produce composites having extraordinary high-strength- and high-modulus-to-weight ratios and other exceptional properties. However, the high cost of producing the high-strength, high-modulus carbon textiles employed in such composites has been a major deterrent to their widespread use, in spite of the remarkable properties exhibited by such composites.
One suggested method of providing high modulus, high strength carbon fibers is described by Johnson et al. in U.S. Letters Pat. No. 3,412,062, entitled "Production of Carbon Fibers and Compositions Containing Said Fibers". Such method comprises heating polyacrylonitrile fiber under tension in an oxidizing atmosphere at a temperature of from 200.degree. C. to 250.degree. C. for a time sufficient to achieve substantially complete permeation of oxygen throughout the fiber, and then carbonizing the oxidized fiber in a non-oxidizing atmosphere to produce a fiber having a high tensile strength and Young's modulus. However, lengthy heat treatment times under oxygen are required by that process, e.g., of the order of at least several hours to 24 hours or more, in order to completely permeate the fiber with oxygen and achieve sufficient stabilization of the fiber structure so that it may be carbonized to produce carbon fibers having properties acceptable for commercial use. Such extended heat treatment times, however, reduce production output and require substantial capital investment, rendering the process unattractive for commercial operations. For this reason, means have been sought for reducing the heat treatment times necessary to stabilize these fibers before they can be carbonized.
According to U.S. Letters Pat. No. 3,529,934, entitled "Process For The Preparation of Carbon Fibers", to Akio Shindo, improved quality carbon fibers can be prepared in high yields from cellulosic, polyvinyl alcohol and acrylic fibers, by heat treating the fibers under tension in an inert atmosphere containing gaseous hydrogen chloride. As in the case of the process suggested by Johnson et al., however, lengthy heat treatments are required in order to produce carbon fibers having properties acceptable for commercial use.
Rostislav Didchenko in U.S. Letters Pat. No. 3,441,378, entitled "Process For The Manufacture of Carbon Textiles", has also suggested that carbon textiles can be obtained from cellulosic textiles in improved yields by heating a cellulosic textile to a temperature up to about 400.degree. C. in an atmosphere containing a hygroscopic gas which reacts with cellulose as a dehydrating agent to form cellulosic intermediates which upon subsequent carbonization yield close to theoretical amounts of carbon. Among the reactive gases mentioned in the patent is hydrogen chloride which, it is said, may be employed together with air as the reactive atmosphere. However, the use of hygroscopic gases like hydrogen chloride with oxygen to effect stabilization of the fiber structure of other carbonizable organic fibers not capable of being dehydrated by such gases, such as polyacrylonitrile, has not been suggested.